Among the numerous proposed and utilized chromatographic techniques are: (1) high performance liquid chromatography (HPLC) with external recycling and (2) simulated moving bed (SMB) chromatography. External HPLC recycling has been known since at least 1974 (R. A. Henry, S. H. Byrne, and D. R. Hudson, J. Chromatographic Sci. 12, 197 (1974)). In external recycling, the chromatographic peaks are caused to cycle through two columns in a figure-of-eight pattern so as to increase the number of theoretical plates. During the recycling procedure, the peaks do not pass through a pump. Thus, this technique is more efficient than closed loop recycling in which the peaks pass through a pump each cycle, causing mixing and cancellation of some of the separation that occurs in the column.
SMB chromatography, invented in the early 1960's by workers at UOP (D. B. Broughton, R. W. Neuzil, J. M. Pharis, C. S. Brearley, Chem. Eng. Progress, 66(9), 70 (1970)), is a continuous process. Feed is continuously injected into the interior of the SMB profile; extract and raffinate are continuously collected; and fresh mobile phase is also added continuously. The entire profile travels around the system. For example, in one possible configuration of a 16 column array, the feed is injected between columns 7 and 8; the mobile phase is injected between columns 16 and 1; the raffinate is collected between columns 11 and 12; and the extract is collected between columns 3 and 4. As the profile moves to the right, all the injection and collection points are switched simultaneously one column to the right. For example, the feed point is between columns 9 and 10; the mobile phase point, between columns 1 and 2; the raffinate point between columns 12 and 13; and the extract point, between columns 4 and 5. The switching occurs periodically at the appropriate times. Because the fluids injected through the feed and mobile phases points are collected at the raffinate and extract points, a steady state develops. A further characteristic of SMB chromatography is that there are four flow rates across the profile. It is also important to stress that classical SMB chromatography is truly continuous: the mobile phase and feed pumps never stop pumping material into the system; and the extract and raffinate lines never stop delivering collected purified material.
The prior art also contains three patents that are relevant: U.S. Pat. No. 4,267,054, U.S. Pat. No. 4,379,751, and U.S. Pat. No. 4,970,002. However, the processes of these patents use closed-loop recycling.